Surface Effect Ships are supported primarily by the lifting force exerted by a slightly pressurized cushion of air contained between the water's surface and the craft.
Under equilibrium conditions, the lift system supplies air at a rate and pressure sufficient to offset the normal leakage under the seals and sidewalls, maintaining an essentially constant pressure. The cushion height is a design variable, dependent primarily on desired sea state operating capabilities.
Any change in cushion volume (because of relative motion between the craft and the water'surface), rate of air supply, or rate of air leakage, will cause a change in the cushion pressure and hence a change in the net lifting force.
When the craft is under way all of the above vary, resulting in high amplitude, random, primarily low frequency, vertical accelerations of the craft.
The craft will follow a path with respect to the surface of the water that can take one of two modes: contouring or platforming. In the contouring mode the craft stays a constant height above the local surface and follows all of its contours. In the platforming mode the craft does not follow the contours of the surface but rather maintains a nearly constant horizontal path regardless of the water's surface. (i.e. negligible pitching motion.)
The mode which the craft will assume is primarily a function of its speed. Below what is termed "hump speed" the craft will usually be in the contouring mode, above that it will usually be in the platforming mode. "Hump speed" is that speed at which the effective drag on the craft goes from a high value to a lower value; the phenomenon is somewhat alike that which occurs when a hydrofoil craft enters the full planing mode of operation.
In the contouring mode, cushion pressure (which is a function of the vehicle's weight and support area) remains essentially constant. At high forward speeds over rough water, the rate of rise and fall of the vehicle as it follows the water's surface can approach several cycles per second. This results in an uncomfortable ride due to the pitching motion induced by the craft following the water's surface; furthermore, it subjects the craft to potentially damaging stresses.
In the platforming mode, the distance between the craft's deck and the water's surface is constantly varying. This results in a constantly varying cushion pressure, which could approach resonance under certain conditions. Further, since the vehicle's vertical motions are of relatively low amplitude, the craft can slam into the tops of the higher waves. This produces an extremely uncomfortable ride as well as subjecting the craft to potentially damaging forces.
The above problems are alleviated by modulating cushion pressure, as is well known in the art.
In the prior art, the air that was vented from the cushion to reduce its pressure was dumped overboard, wasting the energy contained within it; the present invention is a method and means for extracting useful work out of the cushion air that is vented, thereby supplying auxiliary power while reducing fuel consumption.